It's not based on volume as from October 2008 - it's based on circumstances as Stoneyboy points out and where the extension is to be located on the original building for instance distances from common boundaries and/or highways. A highway is not just a road but anything that you can get from a to b for example a bridleway.
The legislation is under the general permitted development order as amended 2008 - class A -
But beware Council's take different interpretations as the legislation is so new that there is no case law to rely on.
You need to submit a certificate of lawfulness to make sure the building is permitted development not requiring planning permission.
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